Gas bubble prevention system for liquid insulated electrical apparatus



May 30, 1961 GAS BUBBLE PREVENTION SYSTEM FOR LIQUID INSULATEDELECTRICAL APPARATUS Filed May 15, 1958 DETRICK ET AL 2,986,593

frzflnffirs, Herberf B. Defr/c/c, KW? H. Weber, /5 5M 732,1,

United States Patent GAS BUBBLE PREVENTION SYSTEM FOR LIQUID INSULATEDELECTRICAL APPARATUS Herbert B. Detrick, Pittsfield, Mass., and Kurt H.Weber, Shaker Heights, Ohio, assignors to General Electric Company, acorporation of New York Filed May 15, 1958, Ser. No. 735,478

6 Claims. (Cl. 174-12) This invention relates to electrical apparatus,and more particularly to an improved means for preventing the formationof gas bubbles in the dielectric liquid of stationary electricalinduction apparatus of the type having an electric device immersed in adielectric liquid which is blanketed by a gas in a sealed container.

Stationary electrical induction apparauts, such as transformers or thelike, are frequently immersed in a dielectric liquid, such as oil. Inorder to prevent contamination of the dielectric liquid by contact withair, the apparatus tanks are frequently sealed, and in order to allowfor thermal expansion of the liquid in the tank, an inner gas space isprovided within the tank contacting the liquid. As an example, a gasfrequently employed in such capacity in electrical apparatus isnitrogen.

Where a gas and liquid are in such contact, some of the gas dissolvesinto the liquid, depending upon the kind of gas and liquid employed andthe temperature and pressure of the fluids. The relationship between theamount of gas that dissolves in the liquid and the pressure of a gas mayin some cases be a simple linear relationship. As an example, oil at 25C. exposed to a nitrogen pressure of one atmosphere will dissolve 8.9%by volume of nitrogen, and when the nitrogen pressure is twoatmospheres, the oil will dissolve 17.8% by volume of nitrogen.

If a closed system of transformer oil blanketed by nitrogen is giventime to reach equilibrium at two atmospheres of pressure, so that theamount of nitrogen dissolved in the oil is 17.8% of the oil volume, andif the pressure is then released, for example, as a result of decreasein oil temperature in response to decrease in load, loss of excitation,or decrease in ambient temperature, the nitrogen Will remain in the oilif the oil is undisturbed. The oil is then said to be in asupersaturated state, since the quantity of nitrogen exceeds theequilibrium quantity of 8.9% corresponding to its new environment at oneatmosphere. If a suflicient time is permitted, the excessive nitrogenwill diffuse out of the oil and eventually the system will restoreitself to equilibrium corresponding to one atmosphere and 25 C., thatis, 8.9% nitrogen by volume. This return to equilibrium may require anextended period of time, depending upon the surface to volume ratio ofthe oil and the amount of motion in the oil due to convection or otherminor disturbances.

If the oil remains undisturbed, the gas will diffuse out of the oil inan orderly manner. There are several operating conditions, however, thatcan cause supersaturated oil to evolve gas in the form of gas bubblesthroughout the oil. For example, such evolution of gas bubbles may arisefrom the agitation of oil by oil circulation pumps, the electrical shockof a transient voltage, or the mechanical shock of a short circuit.While the extent of supersaturation required to produce bubbles isdependent upon variables other than the extent of saturation, it hasbeen found that, with nitrogen in transformer oil, bubble formation canbe expected if the oil 2,986,593 Patented May 30, 1961 is over saturatedby 12% to 20%, i.e., when the pressure differential between gas in theoil and gas above the oil is between 2 and 3 pounds per square inch.

Due to the difference in dielectric constant and dielectric strength ofnitrogen and oil, the formation of nitrogen bubbles in the oil has beenfound to reduce the dielectric strength of the oil by as much astwo-thirds of its electrical strength without gas bubbles. Needless tosay, this reduction of dielectric strength is highly undesirable, andmay result in severe damage to or failure of the transformer.

In some instances a supply of gas, for example, from a pressurizedcylinder of gas, is provided in order to maintain the pressure of thegas cushion within predetermined limits, since it has been found thatsubstantial gas bubble evolution may occur if the range of pressureswithin the transformer tank is above about 2.5 p.s.i.g. In this type ofsystem, which is known as the automatic gas seal type of oilpreservation system, when the gas pressure reaches a maximum desiredlevel the gas is vented to the atmosphere and when the pressure reachesa minimum level more gas is admitted to tank from the gas supply. Whilesuch a system may satisfactorily prevent evolution of gas bubbles if therange between the maximum and minimum pressures within the transformertank is sufliciently narrow, the maintenance of such necessarily narrowlimits may be highly undesirable due to the large quantities of gas thatare normally expended in this type of oil preservation system as aresult of periodic temperature cycling during operation of thetransformer.

It is therefore an object of this invention to provide an improvedsystem for the prevention of gas bubble evolution in electricalapparatus of the type having an electric device immersed in a dielectricliquid.

It is a further object of the invention to provide a gas bubbleprevention system for maintaining a fluid pressure in electric apparatusof the type having electric device immersed in a dielectric liquid, thesystem maintaining the pressure in such apparatus within a suflicientlynarrow range that evolution of gas bubbles in a liquid does not occur,and also not requiring the expenditure of excessive amounts of gas froma gas supply connected to a gas cushion in the apparatus.

Still another object of this invention is to provide a gas bubbleprevention system for electrical apparatus of the type having anelectrical device immersed in a dielectric fluid in a sealed tank andhaving a gas cushion contacting the liquid, the gas for said gas cushionbeing supplied from a pressurized gas supply, said system beingcharacterized in that a narrow range of gas pressure is permitted insaid gas cushion without the excessive expenditure of gas from the gassupply.

In accordance with one aspect of the invention, we provide means forpreventing the evolution of gas bubbles in electrical apparatus of thetype comprising a sealed enclosure containing a dielectric liquid incontact with a gas, and having an electric device immersed in theliquid. The means comprises storage tank means, means for withdrawingthe gas from the enclosure to the storage tank means at a predeterminedmaximum enclosure pressure, and means for releasing gas from the storagetank to the enclosure, when the pressure in the enclosure is below apredetermined pressure which is less than the maximum desired pressure.

In the preferred arrangement of the invention, a supply of pressurizedgas is provided to supply gas to the tank at a minimum pressure, and thegas is released from the storage tank to the enclosure at a pressurebetween the maximum pressure at which gas is withdrawn from theenclosure to the tank and the minimum pressure at which gas is suppliedby the gas supply.

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which we regard as ourinvention, it is believed that theinvention will be better understoodfrom the following description taken in connection withthe accompanyingdrawing.

The single figure in the drawing illustrates a cross sectional view of atransformer and schematically shows the pressure regulation system ofthe invention as it may be connected to the transformer.

Referring now to the drawing, therein is illustrated an electricalapparatus such as a transformer comprising a sealed tank 11 partiallyfilled with a dielectric liquid 12 such as oil. A gas space 13 isprovided in the upper portion of the tank 11, to act as a cushion forvolumetric changes in the oil 12. The transformer core 14 and coils 15are immersed in the dielectric liquid 12, an external connection to thewindings 15 is provided by way of bushings 16 mounted on and extendingthrough the exterior walls or top of the tank 11.

A supply of gas, such as pressurized gas cylinder 20, is connected, byway of regulating valve 21 and conduit 22, to supply gas at apredetermined minimum pressure to the gas space 13 in tank 11. A gasstorage tank 25 is also connected, by way of a regulating valve 26 andconduit 22, to the gas space 13 of the apparatus tank 11, and thestorage tank 25 is also connected to the gas space 13, by a separatepath including a pump 27 and a check valve 28. The pump 27 is arrangedto withdraw gas from the gas space 13 through a conduit 29, and thecheck valve 28 is arranged to prevent gas from flowing back through thepump 27 and conduit 29 to the gas space 13. The regulating valves 21 and26 are arranged to regulate the flow of gas respectively from cylinderand storage tank to the tank 11. A relief valve 30 may be provided inthe conduit 22, for example, in order to relieve excessive pressures inthe apparatus tank 11.

A pressure responsive device, such as a bellows 31 is provided withinthe gas space 13. The bellows 31 is mechanically coupled to suitableswitching means 32 which is electrically connected to effect theenergization of the pump 27 in response to the pressure in the gas space13.

During normal operation of the apparatus 10, the temperature of thedielectric liquid 12 will vary depending, for example, upon theelectrical load supplied by the apparatus and the atmospherictemperature. These temperature variations result in volumetric variationof the liquid, and, hence, variations in the pressure in the gas space13. The pressure responsive means 31 is adjusted to eifect theenergization of the pump 27, by means of the switch means 32, at apredetermined maximum pressure within the gas space 13. When the pump 27is energized, gas is withdrawn by way of the conduit 29, pump 27 andcheck valve 28 to the storage tank 25. The withdrawal of gas from thegas space 13 continues until the energization of the pump 27 is stoppedin response to detection of a second predetermined pressure within thegas space 13 by the pressure responsive means 31. The secondpredetermined pressure at which the operation of the pump ceases is ofcourse lower than the maximum predetermined pressure at which theoperation of the pump commences. The check valve 28 serves to preventpassage of the gas from the storage tank 25 back through the pump 27 andconduit 29 to the gas space 13.

When the pressure in the gas space 13 drops below a third predeterminedpressure (below the pressures at which the pump 27 is energized andde-energized) the regulating valve 26 opens to permit release of gasfrom the storage tank 25 to the gas space 13 by way of conduit 22. Thediiferential pressure between the maximum predetermined pressure and thepressure at which valve 26 opens is sufiiciently narrow that gas bubbleevolution will not occur in the apparatus.

If the pressure within the gas space 13 drops below a predeterminedminimum value (less than the pressure of opening of the regulating valve26) such as may occur if insufiicient gas is present in the storage tank25, the regulating valve 21 opens to permit release of gas from thepressurized gas cylinder 20 to the gas space 13 by way of the conduit22. The relief valve 30 may be set to relieve pressure in the gas space13 at a predetermined pressure above the maximum desired operatingpressure within the gas space.

As an example of typical settings of the valve and pressure responsivedevice to prevent evolution of gas in the liquid 12, the pressureresponsive means 31 may be set to energize the pump 27 at a pressure of1.5 p.s.i.g. and de-energize the pump at a pressure of 1.0 p.s.i.g. Theregulating valve 26 may be set to open when the pressure in the gasspace 13 falls below 0275 p.s.i.g., and the regulating valve 21 may beset to open when the gas pressure in the gas space 13 falls below 0.5p.s.i.g. With these settings, the pressure in the gas space 13 ismaintained between the pressures 0.5 p.s.i.g. and 1.5 p.s.i.g., adiiferential of one pound per square inch in pressure, which issufiicient to prevent the evolution of bubbles in the liquid 12. Therelief valve 30 may be set to relieve pressure in the gas space 13 at2.5 p.s.i.g. in the event of sudden buildup of pressure in the tank 11.It will be understood, of course, that the above example is purelyillustrative, and other pressure settings of the valve and pressureresponsive device may be employed to attain a desired pressure rangewithin the gas space.

Since variations in the pressure in gas space 13 are compensated for bywithdrawing gas to the tank 25 and releasing gas from the tank 25, thepresent invention provides a fluid pressure regulating system having theadvantages of the previously employed automatic gas seal type of oilpreservation system without having the disadvantage of the largeexpenditure of gas resulting from pressure variations within the gasspace 13, and the expenditure of gas is not dependent upon thenarrowness of range of pressures within the gas space.

It will be understood, of course, that while the form of the inventionherein shown and described constitutes the preferred embodiment of theinvention, it is not intended herein to illustrate all of the possibleequivalent forms or ramifications thereof. It will also be understoodthat the words employed are words of description rather than oflimitation, and that various changes may be made without departing fromthe spirit and scope of the invention herein disclosed, and it is aimedin the appended claims to cover all such changes as fall within the truespirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. Means for preventing the evolution of gas bubbles in electricalapparatus of the type comprising a sealed enclosure containing adielectric liquid in contact with a gas and an electric device immersedin said liquid, said means comprising storage tank means, pump meansoperatively connected between said tank means and enclosure to withdrawgas from said enclosure to said tank means when the pressure in saidenclosure is between a predetermined maximum value and a second value, acheck valve operatively connected between said tank means and pump meansto prevent gas compressed in said tank means from flowing back throughsaid pump to said sealed enclosure, and valve means operativelyconnected between said tank means and enclosure to release gas from saidtank means to said enclosure when the pressure in said enclosure isbelow a predetermined value less than said predetermined maximum value.

2. Means for preventing the evolution of gas bubbles in electricalapparatus of the type comprising a sealed enclosure containing adielectric liquid in contact with a gas and an electric device immersedin said liquid, said means comprising storage tank means, pump meansconnected to P p as from said enclosure to said tank means, means fordetecting the fluid pressure within said enclosure, said pressuredetecting means being connected to energize said pump means to withdrawgas from said enclosure when the pressure therein is between apredetermined maximum value and a second value, a check valveoperatively connected between said tank means and pump means to preventgas compressed in said tank means from flowing back through said pump tosaid sealed enclosure, and valve means operatively connected betweensaid tank means and said enclosure to release gas to said enclosure whenthe pressure therein is below a predetermined value less than saidpredetermined maximum value, the difierential between predeterminedpressures being sufiiciently narrow to prevent the evolution of gasbubbles in said liquid.

3. Means for preventing the evolution of gas bubbles in electricalapparatus of the type comprising a sealed enclosure containing adielectric liquid in contact with a gas and an electric device immersedin said liquid, said means comprising storage tank means, pump meansconnected to pump gas from said enclosure to said tank means, means fordetecting the fluid pressure within said enclosure, said detecting meansbeing operatively connected to eflect the energization of said pumpmeans when the pressure within said enclosure reaches a firstpredetermined value and to continue the energization of said pump meansuntil the pressure within said enclosure drops to a second predeterminedvalue, means for preventing return flow of gas from said storage tankmeans to said enclosure through said pump means, and valve meansconnected between said tank means and enclosure to release gas from saidtank to said enclosure when the pressure in said enclosure is below athird predetermined pressure less than said second predeterminedpressure, the differential between said first and third predeterminedpressures being sufficiently narrow to prevent the evolution of gasbubbles in said liquid.

4. Means for preventing evolution of gas bubbles in the dielectricliquid of electrical apparatus of the type having an electric deviceimmersed in said liquid in a sealed enclosure, and in which saidenclosure also contains a gas in contact with said liquid, said meanscomprising storage tank means, compressor means responsive to fluidpressure within said enclosure to pump gas from said enclosure to saidtank to maintain the fluid pressure within said enclosure between apredetermined maximum value and a second value, means for preventingreturn flow of gas from said storage tank means to said sealed enclosurethrough said compressor means, and valve means responsive to fluidwithin said enclosure to release gas from said storage tank means tosaid enclosure to maintain the fluid pressure within said enclosureabove a predetermined minimum value, the differential between saidminimum and maximum pressures being sufiiciently narrow to prevent theevolution of gas bubbles in said liquid.

5. An electrical transformer comprising a sealed enclosure containing atransformer core and winding coil, a dielectric liquid in contact withsaid core and winding partially filling said enclosure, a gas that issoluble in said liquid filling the remainder of said enclosure, meansfor preventing the evolution of gas bubbles in said liquid comprising asupply of pressurized gas connected to said enclosure, a gas storagetank, a compressor pump connected to said enclosure and storage tank topump gas from said enclosure to said storage tank, means for detectingthe fluid pressure within said enclosure, the pressure detecting meansbeing connected to energize said compressor pump to withdraw gas fromsaid enclosure when the pressure therein attains a pre-determinedmaximum value, a check valve operatively connected between said storagetank and compressor pump to prevent gas compressed in said storage tankfrom flowing back through said pump to said enclosure, a valveoperatively connected between said supply of pressurized gas and saidenclosure to release gas to said enclosure when the pressure therein isbelow a pre-determined minimum value less than said predeterminedmaximum value, and other valve operatively connected between saidstorage tank and enclosure to release gas from said storage tank to saidenclosure when the pressure therein below a pre-determined valueintermediate said maximum and minimum pre-determined values, thediiferential between the maximum and intermediate pressures beingsufficiently narrow to prevent the evolution of gas bubbles in saidliquid.

6. A transformer as recited in claim 5 in which said means for detectingthe fluid pressure within said enclosure comprises a bellows locatedentirely within said enclosure and mechanically coupled to switchingmeans which is electrically connected to effect the energization of saidcompressor pump.

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